Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units (host adapters), disk drives, and disk interface units (disk adapters). Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass. and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., U.S. Pat. No. 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky et al., and U.S. Pat. No. 5,857,208 to Ofek, which are incorporated herein by reference. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units. The logical disk units may or may not correspond to the actual disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data stored therein.
In some instances, it may be desirable to copy data from one storage device to another. For example, if a host writes data to a first storage device, it may be desirable to copy that data to a second storage device provided in a different location so that if a disaster occurs that renders the first storage device inoperable, the host (or another host) may resume operation using the data of the second storage device. Such a capability is provided, for example, by the Remote Data Facility (RDF) product provided by EMC Corporation of Hopkinton, Mass. With RDF, a first storage device, denoted the “primary storage device” (or “R1”) is coupled to the host. One or more other storage devices, called “secondary storage devices” (or “R2”) receive copies of the data that is written to the primary storage device by the host. The host may interact directly with the primary storage device, but any data changes made to the primary storage device are automatically provided to the one or more secondary storage devices using RDF. The primary and secondary storage devices may be connected by a data link, such as an ESCON link, a Fibre Channel link, and/or a Gigabit Ethernet link. The RDF functionality may be facilitated with an RDF adapter (RA) provided at each of the storage devices.
Data transfer among storage devices, including transfers for data replication or mirroring functions, may involve various data synchronization processing and techniques to provide reliable protection copies of data among a source site and a destination site. In synchronous transfers, data may be transmitted to a remote site and an acknowledgement of a successful write is transmitted synchronously with the completion thereof. In asynchronous transfers, a data transfer process may be initiated and a data write may be acknowledged before the data is actually transferred to directors at the remote site. Asynchronous transfers may occur in connection with sites located geographically distant from each other. Asynchronous distances may be distances in which asynchronous transfers are used because synchronous transfers would take more time than is preferable or desired.
In an active/active storage system, if there are multiple interfaces to a storage device, each of the interfaces may provide equal access to the storage device. With active/active storage access, hosts in different locations may have simultaneous read/write access via respective interfaces to the same storage device. For both synchronous and asynchronous transfers within active/active systems, it may be desirable to maintain a proper ordering of writes such that any errors or failures that occur during data transfer may be properly identified and addressed such that, for example, incomplete data writes be reversed or rolled back to a consistent data state as necessary, Reference is made, for example, to U.S. Pat. No. 7,475,207 to Bromling et al. entitled “Maintaining Write Order Fidelity on a Multi-Writer System,” which is incorporated herein by reference, that discusses features for maintaining write order fidelity (WOF) in an active/active system in which a plurality of directors (i.e. controllers and/or access nodes) at geographically separate sites can concurrently read and/or write data in a distributed data system.
For further discussions of data ordering and other techniques used for synchronous and asynchronous data replication processing in various types of systems, including types of Remote Data Facility (RDF) systems produced by EMC Corporation of Hopkinton, Mass., reference is made to, for example, U.S. Pat. No. 7,613,890 to Meiri, entitled “Consistent Replication Across Multiple Storage Devices,” U.S. Pat. No. 7,054,883 to Meiri et al., entitled “Virtual Ordered Writes for Multiple Storage Devices,” U.S. Patent App. Pub. No. 2006/0069893 A1 to LeCrone et al., entitled “Host Implementation of Triangular Asynchronous Replication,” and U.S. patent application Ser. No. 12/080,027 to Meiri et al., filed Mar. 31, 2008, entitled “Active/Active Remote Synchronous Mirroring,” which are all incorporated herein by reference and are assigned to the assignee of the present application.
U.S. Pat. No. 7,779,291 to Yoder et al., entitled “Four Site Triangular Asynchronous Replication,” which is incorporated herein by reference, discloses features of a system to provide four site asynchronous replication among multiple destinations (e.g., storage devices) that store recovery data for a data center. The remote destinations may maintain maps that associate data being provided to a local destination with data stored on the remote destinations. The storage controllers at each of the remote destinations may be used to maintain and control replication features of the system using described techniques and mechanisms. In various circumstances, however, four site replication features controlled by respective storage controllers may be undesirably complex to maintain and subject to multiple failure points.
Accordingly, it would be desirable to provide an effective and efficient system to address issues like that noted above for providing data replication features in a distributed storage system, particularly in an active/active storage system.